1. Field of the Invention
The present invention relates to ink jet printing apparatus and, more particularly, to a drop on demand type ink jet printhead having a modulatable cover plate which controls the ejection of droplets of ink therefrom.
2. Description of Related Art
Printers provide a means of outputting a permanent record in human readable form. In general, printing techniques may be divided into two types--impact printing techniques and non-impact printing techniques. A very popular non-impact printing technique is generally classified as ink jet printing. Ink jet printing systems use the ejection of tiny droplets of ink to produce an image. The devices produce highly reproducible and controllable droplets so that a droplet may be printed at a location specified by digitally stored image data.
Most ink jet printing systems commercially available may be generally classified as either a "continuous jet" type ink jet printing system where droplets are continuously ejected from the printhead and either directed to or away from the paper depending on the desired image to be produced or as a "drop on demand" type ink jet printing system where droplets are ejected from the printhead in response to a specific command related to the image to be produced.
In a continuous jet type ink jet printer, a pump continuously supplies ink to a nozzle assembly where the pumping pressure forces the supplied ink to be ejected therefrom in a continuous stream. The nozzle assembly includes a piezo crystal continuously driven by an electrical voltage, thereby creating pressure disturbances that cause the continuous stream of ink ejected therefrom to break up into uniform droplets of ink. The droplets acquire an electrostatic charge due to the presence of an electrostatic field established close to the ejection orifice. Using high voltage deflection plates, the trajectory of selected ones of the electrostatically charged droplets can be controlled to hit a desired spot on a sheet of paper. The high voltage deflection plates can also deflect unselected ones of the electrostatically charged droplets away from the sheet of paper and into a reservoir for recycling purposes. Due to the small size of the droplets and the precise trajectory control, the quality of continuous jet type ink jet printing systems can approach that of formed-character impact printing systems. However, the primary drawback to continuous jet type ink jet printing systems is that fluid must be continuously jetting, even when little or no printing is required. This requirement degrades the ink and decreases reliability of the printing system.
Due to this drawback, there has been increased interest in those ink jet printing systems in which droplets of ink are ejected from the printhead by electromechanically inducing pressure waves in the fluid. In this type of printing system, a volumetric change in the fluid is induced by the application of a voltage pulse to a piezoelectric material which is directly or indirectly coupled to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid, thereby causing the ejection of a droplet therefrom. Since the voltage is applied only when a droplet is desired, these types of ink jet printing systems are referred to as drop on demand type ink jet printing systems.
A typical drop on demand type ink jet printing system is disclosed in U.S. Pat. No. 3,946,398 to Kyser et al. In Kyser et al., a pressure plate formed from two transversely expandable piezoelectric plates is utilized as the upper wall of an ink-carrying pressure chamber. By applying a voltage across the piezoelectric plates, the pressure plate flexes inwardly into the pressure chamber, thereby causing a fluid displacing volumetric change within the chamber. Another typical drop on demand type ink jet printing system is disclosed in U.S. Pat. No. 4,536,097 to Nilsson. In Nilsson, an ink jet channel matrix is formed using a series of piezoelectric strips disposed in spaced parallel relationship with each other and covered by a plate on both sides. One plate is constructed of a conductive material and forms a shared electrode for all of the strips of piezoelectric material. On the other side, electrical contacts are used to electrically connect channel defining pairs of the strips of piezoelectric material. When a voltage is applied to the two strips of piezoelectric material which define a channel, the strips become narrower and higher such that the enclosed cross-sectional area of the channel is enlarged and ink is drawn into the channel. When the voltage is removed, the strips return to their original shape, thereby reducing the channel volume and ejecting ink therefrom. Other, albeit shear mode, ink jet printing systems which utilize separate sections of a piezoelectric material to form individual actuator walls for an ink-carrying channel are disclosed in U.S. Pat. Nos. 4,879,568 to Bartky et al. and 4,887,100 to Michaelis et al.
The major drawback to drop on demand type ink jet printing systems such as those disclosed in Kyser et al., Nilsson, Bartky et al. and Michaelis et al. is the difficulty in manufacturing an ink jet printhead, particularly a large array or high density ink jet printheads, in such configurations. Each of these configurations utilize a separate actuator piece for each channel. Accordingly, to construct such a printhead, a large number of individual parts must be used to assemble the channel array. Electrical contacts are either separately attached to each actuator before assembly or attached after the channel array is assembled, either of which is a very time consuming operation. Due to the large number of steps required to assemble such a printhead, the manufacture of such a printhead, in particular one having a nozzle density greater than 100 nozzles per inch has proven difficult, as well as often cost prohibitive, in practice.
The present invention represents a significant departure from prior drop on demand type ink jet printheads in that an inactive channel array having no channel actuators incorporated therein is utilized. As a result, the construction of the channel array has been simplified dramatically in comparison with prior systems. Furthermore, by eliminating the use of an acoustic wave to control the ejection of ink from the printhead, enhanced modulation of the droplet has become possible.